Numerical Abnormalities: Overview of Trisomies and Monosomies

What are numerical chromosome abnormalities?

Numerical abnormalities are a type of chromosome defect. These types of birth defects
occur when there is a different number of chromosomes in the cells of the body from
what is usually found. So, instead of the typical 46 chromosomes in each cell of the
body, there may be 45 or 47 chromosomes. Having too many or too few chromosomes may
cause health problems or birth defects.

What are trisomies?

The term "trisomy" is used to describe the presence of 3 chromosomes, rather than
the usual pair of chromosomes. For example, if your baby is born with 3 #21 chromosomes,
rather than the usual pair, your baby would be said to have "trisomy 21." Trisomy
21 causes Down syndrome. Other examples of trisomy include trisomy 18 and trisomy
13. Again, trisomy 18 or trisomy 13 simply means there are 3 copies of the #18 chromosome
(or of the #13 chromosome) present in each cell of the body, rather than the usual
pair.

What are monosomies?

The term "monosomy" is used to describe the absence of one member of a pair of chromosomes.
Therefore, there is a total of 45 chromosomes in each cell of the body, rather than
46. For example, if your baby is born with only one X chromosome, rather than the
usual pair (either two X's or one X and one Y chromosome), your baby would be said
to have "monosomy X." Monosomy or partial monosomy is the cause of certain diseases
such as Turner syndrome and Cri-du-Chat syndrome.